1. Technical Field
The present invention relates to the technical field of semiconductor materials and specifically to a method for cleaning & passivating gallium arsenide (GaAs) surface autologous oxide and depositing Al2O3 dielectric.
2. Description of Related Art
With the fast development of the microelectronics industry and optoelectronics industry, demands on semiconductor lasers, optical receivers used in fiber optic communications, and high-speed & high-frequency semiconductor devices made of III-V compound semiconductor materials such as GaAs have increased, so studies on III-V compound semiconductor materials such as GaAs are always a focus in the academic as well as industrial fields. Due to its high electron mobility, big forbidden bandwidth (1.43 eV) and low charge carrier concentration, GaAs intrinsic material is widely used in high-speed devices and high-temperature environments. GaAs are believed to be the best possible channel material that could replace silicon (Si) for the CMOS device in the Post-Silicon era. However, due to exposure to the air, GaAs tends to be oxidized into a series of complicated oxides and arsenic elementary substances, which causes many surface defects in the GaAs material and makes it unusable in microelectronic and optoelectronic devices. For a long time, the incapability of manufacturing GaAs with a clean surface and lack of proper dielectric material for passivizing GaAs were the two major problems that impede the large-scale application of GaAs material.
In the 1960s and 1970s, in accordance with the idea of oxidating Si to produce high-quality SiO2 film, researchers made a lot of efforts to study how to use the autologous oxide of GaAs as a passivation dielectric, but all failed. Through in-depth research, it was found that the oxide of arsenic is unstable and that the density of the interfacial states between the autologous oxide layer and GaAs is quite high, which plays a role as the scattering center and nonradiative recombination center for charge carriers, reduces the mobility of charge carrier, and causes Fermi level pinning, thus seriously affecting the electrical and optical properties of devices.
After the regular cleaning of GaAs, the GaAs surface autologous oxide cannot be completely cleaned, and after cleaning, the GaAs sample will be inevitably exposed to the air before the next process. The clean GaAs surface has active chemical properties, and the surface of the GaAs sample will have the following reactions with the oxygen in the air:3O2+2GaAs=As2O3+Ga2O3  (1)4O2+2GaAs=As2O5+Ga2O3  (2)As2O3+2GaAs=Ga2O3+4As  (3)3As2O5+10GaAs=5Ga2O3+16As  (4)
Therefore, various components such as Ga2O3, As2O3 and As will be generated on the GaAs surface.
In 1987, Sandroff et al first proposed the sulfur passivation method for passivating GaAs surface with a sulfide solution. They passivized an AlxGa1-xAs/GaAs heterojunction bipolar transistor (HBT) device with Na2S.9H2O solution, thus the HBT low-current amplification factor was increased 60 times, and after passivation, the photoluminescence (PL) intensity is increased 250 times. The mechanism of the wet process of sulfur passivation is to generate a dense passivation layer mainly composed of GaxSy and AsxSy through chemical reaction of the S2− ions with the GaAs autologous oxide and arsenic elementary substance. The sulfur passivation method can effectively remove the unstable oxide from the surface and generate a sulfide layer, which can also inhibit the surface from being oxidized again, reduce the surface state density and surface recombination rate, and basically achieving the effect of reducing the surface state density (electrical passivation) and increasing stability (chemical passivation). In recent years, various sulfur passivation methods for GaAs materials have been invented and adopted, which have significantly improved the surface properties of GaAs.
In general, the (NH4)2S passivation solution capable of generating a large amount of S2− and HS− ions is used for GaAs. However, due to its fast speed of reaction, it will cause severe corrosion to the surface of GaAs and generate many pits. Moreover, the water in the solution will cause the reoxidation of GaAs in the presence of oxygen, and the generated sulfide layer will be thin, which is disadvantageous to the follow-up procedures. A clean and oxygen-free GaAs substrate surface is the precondition for developing high-quality films. In view of this, the mixture of CH3CSNH2, ethanol and gradually hydrolyzed ammonia water is used to generate HS− and S2− ions so as to passivize the GaAs surface and generate a denser and thicker layer of sulfide which will separate the GaAs from the outside environment. Since the relative dielectric coefficient of alcoholic solution is much lower than that of water, the electrostatic adsorption capacity of HS− and S2− ions and the GaAs surface is increased. The alkaline environment of ammonia water (PH>7) can promote the generation of HS− and S2− ions through hydrolysis, which will help the S form covalent bonds with the GaAs surface atoms, thus the passivized GaAs surface has a lower density of interfacial states and less corroded pits.
Although sulfur passivation has eliminated most autologous oxides and elementary arsenic substances on the GaAs surface, the sulfides generated through passivation, especially As—S will still form a new defect level in the GaAs forbidden band, thus the Fermi level pinning on the GaAs surface still cannot be eliminated. Under a temperature of about 350 C, the As—S bond will decompose and disengage from the GaAs surface, while the Ga—S bond is still stable under the temperature of 460 C. On the other hand, in the atomic layer deposition (ALD) process, self-cleaning occurred on the reaction source and the deposited substrate surface was gradually recognized and utilized, which refers to the reaction between the reaction source absorbed on the substrate surface and the oxide on the substrate, and in this way, substances such as oxides on the substrate will be cleaned. The present invention uses the chemical reaction of trimethyl aluminum (TMA) with AsxSy, GaxSy, AsxOy, GaxOy and As as well as the thermal effect in the ALD process to make the residual sulfide and autologous oxide volatilize and disengage from the GaAs surface, so that a clean substrate surface can be obtained before deposition of the Al2O3 film dielectric.
In accordance with Moore's Law, with continuous reduction of the device size, the working speed will increase and power consumption will also reduce. The trend of continuous reduction of the device size is also reflected on the decrease of the gate oxide layer thickness (Tox) of the metal-oxide-semiconductor field effect transistor (MOSFETS). Because quantum tunneling will cause the grid leakage current to have an exponential rise with the decrease of the gate oxide thickness, when the gate oxide thickness is less than 2 nm, quantum tunneling will cause a fast increase of grid leakage currents, which will ultimately cause performance degradation of the device.
In accordance with the capacitance formula of parallel plate capacitors
      C    =                            ɛ          r                ·                  ɛ          o                ·        S            t        ,in order to increase the capacitance density of grid capacitors, materials with a higher dielectric constant (high-k material) can be used to replace conventional SiO2 as the gate dielectrics. Obviously, in order to obtain the same grid capacitance, corresponding thicknesses of SiO2 and high-k material should satisfy the following relation:
            t              high        -        k                    t              SiO        2              =            ɛ              high        -        k                    ɛ              SiO        2            
Where, thigh-k, tSiO2, ∈high-k and ∈SiO2 represent the film thickness of high-k material, the thickness of SiO2 film, and the dielectric constants of high-k material and the dielectric constant of SiO2 respectively. Under a certain capacitance density of the grid capacitor, the leakage current has significantly reduced after using high-k material as the gate dielectric compared to silica.
Among various high-k materials, from the perspective of material properties and electrical properties, Al2O3 is a high-quality dielectric material. Al2O3 has high a forbidden bandwidth (˜9 eV), high breakdown field strength (5-10 MV/cm) and great thermal stability, and can maintain an amorphous state after high-temperature processing and treatment. Materials such as Al2O3, Si and GaAs have stable interfaces, and Al2O3 has a good blocking effect toward sodium, boron and phosphorus, and also has strong radiation resistance capacity. Therefore, Al2O3 is often used as the passivation film and dielectric material, which can help increase and improve the performance and reliability of the device.